1. Technical Field of the Invention
The present invention relates generally to an angular shift determining apparatus designed to determine an angular shift of a central axis of radiation of radar waves from an obstacle detection system installed in an automotive vehicle for determining the position of a target accurately.
2. Background of Related Art
Automotive obstacle recognition systems are known in the art which are designed to transmit radar waves such as light waves or electromagnetic waves and receive a signal reflected from a detectable zone to determine the distance to and angular direction or azimuth of an object present ahead of the vehicle. Such systems are employed in cruise control which keeps an intervehicle distance constant and/or collision alarm control which measures the distance to an obstacle such as a preceding vehicle and issues an alarm when the measured distance falls in a warning range.
The physical installation of the above automotive obstacle detection systems in vehicles requires precise alignment of a central axis of radiation of radar waves with a longitudinal center line of the vehicle. The misalignment therebetween will result in an error in determining an angular direction of an obstacle tracked by the system, which may cause vehicles traveling on an adjacent traffic lane to be recognized as traveling on the same traffic lane as a system-equipped vehicle, and vice versa. The exact alignment of the central axis of radar wave radiation with the longitudinal center line of the vehicle requires fine adjustment in physical installation of the automotive obstacle detection system, which undesirably consumes the time of the operator.
FIG. 15(a) illustrates for the case where a preceding vehicle 93 is traveling along the longitudinal center line of a system-equipped vehicle 91 (hereinafter, referred to as a system vehicle) at a constant intervehicle distance. If the central axis of radar wave radiation from the system vehicle 91 is shifted, as indicated by numeral 95, from the longitudinal center line of the system vehicle, it will cause the preceding vehicle 93 to be detected in error at a location .theta.m away from the longitudinal center line of the system vehicle.
The angular shift of the central axis of radar wave radiation may be determined by a relative position of an object such as the preceding vehicle 93 existing just ahead of the system vehicle. The inventors of this application have, however, found that errors in determining of the angular shift of the central axis of radar waver radiation will occur in the following cases.
(1) When the preceding vehicle 93 is, as shown in FIG. 15(b), shifted in lateral direction from the system vehicle 91, that is, when the preceding vehicle 93 is traveling with a lateral offset from the longitudinal center line of the system vehicle 91 (which will be referred to below as offset traveling), it will cause an apparent angular shift of central axis of radar wave radiation 95 to be produced.
(2) When the preceding vehicle 93 is, as shown in FIG. 16, traveling on the same lane as the system vehicle 91, but on a curved road, it will cause an angular error .theta. to be produced even if the central axis of radar wave radiation 95 aligns with the longitudinal center line of the system vehicle 91.